Sliding surfaces and displacement rates of extremely-slow landslides: reliability of inclinometer measurements

Movements of extremely-slow landslides may be detected only with instrumentation providing measurements with adequate precision and accuracy. In fact, if a reliable estimate of the velocity is requested in a short time (e.g. some months) by using displacements of only few millimeters, the errors that affect the measurements should be at least one order of magnitude smaller. Otherwise, a specific data processing has to be defined in order to reduce their influence on the displacement evaluation. This paper describes the data processing procedure that was effectively applied to detect the sliding surfaces and to estimate the displacement rates with inclinometer measurements collected in two extremely-slow landslides by using different probes. The procedure includes the identification of: precision, accuracy, interval of integration, and validation of results. The procedure was firstly defined and validated to study the V70 landslide, in the Isarco valley in Northern Italy, that involves a motorway viaduct at the toe of the slope. Then it was applied to an extremely-slow landslide, located on the south-eastern slope of the Monteverde/Gianicolo Hill in Rome (Italy), where a sliding surface with displacements at a rate of 1 mm/year was identified only after one year of measurements.